In a turbocharged engine, a turbine is disposed on an exhaust passage, and a compressor is disposed on an intake passage. The turbine is rotated by an exhaust flow discharged from a combustion chamber of the engine. With this, the compressor directly coupled to the turbine is rotated, and this increases the amount of air supplied to the combustion chamber. A problem in this type of turbocharger is that so-called surging tends to occur especially at the time of deceleration.
FIG. 13 is a compressor map showing a range where the compressor can perform supercharging. The compressor map includes a surging line L, and a region located at a low flow rate side of the surging line L is a surging region. When an operating point P0 plotted by a compressor passing flow rate and a pressure ratio between a pressure upstream of the compressor and a pressure downstream of the compressor (hereinafter referred to as a “compressor pressure ratio”) is located in the surging region, surging occurs, that is, an intake flow vibrates in upstream and downstream directions of the intake passage with the generation of noises.
For example, when a throttle valve disposed on the intake passage is closed at the time of the deceleration, the exhaust flow supplied to the turbine decreases. However, since the turbine continues to rotate by inertial force for a while, the compressor coupled to the turbine also continues supercharging. As a result, supercharged air ejected from the compressor toward a downstream side is stopped by the throttle valve, so that the pressure between the compressor and the throttle valve is maintained for a while. In contrast, the compressor passing flow rate decreases since the throttle valve is closed.
To be specific, the compressor pressure ratio is maintained high, whereas the compressor passing flow rate decreases. In this case, the operating point of the compressor tends to move to the surging region, and the surging occurs.
To suppress the surging, it is known that: the intake passage is provided with a bypass passage that connects a portion of the intake passage which portion is located upstream of the compressor and a portion of the intake passage which portion is located downstream of the compressor; and a bypass valve configured to open and close the bypass passage is provided. For example, PTL 1 discloses that the surging is suppressed in such a manner that by opening the bypass valve at the time of the deceleration, that is, by opening the bypass valve when the throttle valve is closed, the pressure between the compressor and the throttle valve is released through the bypass passage to an upstream side of the compressor, and this decreases the compressor pressure ratio.
The bypass valve of PTL 1 is opened when the pressure downstream of the throttle valve becomes a negative pressure. To be specific, when the throttle valve is closed, such as at the time of the deceleration, the bypass valve is opened.